The present invention relates to a bias control apparatus for automatically controlling a bias voltage to an optimum bias voltage in a tape recorder for recording an analog signal on a magnetic tape.
In a tape recorder for recording an analog signal, for example, an audio signal, a given bias current is applied to the recording head. A magnitude of the bias current is selected such that a single tone from 400 Hz to 1 kHz is recorded at a recording level, for example, from -20 dB to -30 dB with respect to a saturation level of magnetic tape used, and the bias current is adjusted so as to have a maximum of a playback output level of the single tone. The magnitude of the bias current so controlled is called a peak bias or an optimum bias current. The magnitude of the optimum bias current depends on an electric-magnetism converting characteristic of the magnetic tape, a characteristic of the recording head, a transporting speed of the tape, and other conditions of the measurement. Wear of a tape sliding portion of the head often causes the width of a head gap to change and/or a cross sectional area of the head core to reduce. As a result, a reluctance in the magnetic circuit of the recording head and an impedance of the magnetic head gradually change in the course of the use of the tape recorder. Therefore, even when the bias voltage applied to the recording head is fixed, a bias magnetic field around the head gap changes with wear of the head. Thus, the magnitude of the optimum bias current changes also by the wear of the recording head through its use.
As described above, the optimum bias current changes depending on a variety of conditions as mentioned above. In order to record the audio signal always under a good condition, it is desirable to make the control for the optimum base current, before the recording. A known method of the bias control is categorized into a manual method by the rotation of a variable resistor and an automatic method by using a microprocessor. Both the methods make use of the fact that a maximum point (peak bias point) in the level of the reproduced single tone or a point in the vicinity of the peak bias point corresponds to the optimum bias point. When making use of this principle, little difficulty is encountered in the manual bias control method but much difficulty is encountered in the automatic bias control method. The difficulty is that when a playback level is below the maximum point of the playback level, the judgement must be made as to whether the small playback level is due to an over-bias or an under-bias. To solve this problem, the conventional automatic bias control device using the microprocessor adopts a relatively complicated control process.
Much study was made to find a simpler, more reliable automatic bias control method for the optimum bias current which may be substituted for the complicated digital control by the microprocessor, and the present invention have succeeded in finding a servo system which can automatically control the bias current to the optimum value.